Process for producing components having regions of differing ductility

ABSTRACT

The invention relates to a process for producing sheet steel components having regions of differing ductility, in which either a sheet metal plate composed of a hardenable steel alloy is used to produce a component by deep-drawing and the deep-drawn component is then at least partially austenitized by a heat treatment and subsequently quench hardened in a die or the plate is at least partially austenitized by a heat treatment and shaped in the hot state, and is quench hardened during or after this, with the sheet metal plate having a zinc-based cathodic corrosion protection coating, characterized in that in regions of a desired higher ductility of the component, at least one additional sheet is attached to the plate, situated so that during the heat treatment, the plate is heated to a lesser degree there than in the remaining region.

FIELD OF THE INVENTION

The invention relates to a process for producing sheet steel components.

BACKGROUND OF THE INVENTION

In automotive construction, the use of sheet metal products—preferably composed of sheet steel—that vary in thickness and material quality is on the rise. This makes it possible to reduce the weight of vehicle body components while adapting the material used to their functions. Body components of this kind include, for example, A, B, and C pillars, bumpers and their cross members, roof frames, side impact members, exterior body parts, etc.

In this connection, the prior art uses so-called tailored blanks. These are plates that are welded together out of a plurality of sheet metal pieces with the same or different sheet thicknesses and material qualities. The use of so-called patchwork blanks is also known. These are plates of varying thicknesses and material qualities that are placed parallel to one another.

In the latter process, the sheets are placed onto each other and then joined to each other, in particular by spot welding.

Patchwork blanks have the disadvantage that the spot-welded connections are subjected to high stresses during shaping and can sometimes even fracture. In addition, the gap that is present between the sheet metal layers can lead to corrosion problems; controlling these requires an expensive sealing treatment. Furthermore, the transition between the individual thickness regions is relatively abrupt in both tailored blanks and patchwork blanks. This can result in undesirable stress peaks in the immediate transition region.

Although a significant weight reduction is achieved with tailored and patchwork blanks, the corrosion protection is relatively expensive.

DE 100 11 589 A1 has disclosed a process for producing sheet metal plates that are multilayered in some regions; in this process, a smaller plate is joined to a larger plate through application of an intermediate layer of adhesive. Before the plates are joined, a powder coating process is used to apply the intermediate layer of adhesive to the smaller plate. Preferably, the smaller plate is covered over its entire area with a powdered resin that constitutes the intermediate layer of adhesive and then the plates, which have been jointly cut to size, are pressed together while undergoing a temperature treatment to form a composite and are then cooled before undergoing a joint deep-drawing process.

DE 10 2004 031 797 A1 has disclosed a process for producing a shaped, locally reinforced sheet metal component in order to produce a corresponding sheet metal component; according to this process, before or during the shaping process, a reinforcing plate is fastened to a base plate by means of a soldered connection; a nickel-based solder material is used to produce the soldered connection.

DE 100 49 660 A1 has disclosed a process for producing locally reinforced, shaped sheet metal parts in which the base plate of the structural component is joined in a defined way to the reinforcing sheet metal layer in the flat state and this patched composite plate is then shaped as a unit. In order to improve the production process with regard to the results and the process product as well as to relieve the strain on the mechanisms that carry out the process, the composite sheet is heated to at least 800 to 850° C. before the shaping, inserted into the die quickly, rapidly shaped while still in the hot state, and then cooled in a defined way with mechanical fixing of the shaped state through contact with the shaping die, which is equipped with forced cooling from the inside. It is particularly important here for the temperature to pass through the range from 800 to 500° C. along a defined temperature ramp; the step of joining the reinforcing plate to the base plate can be integrated into the shaping process by hard-soldering the pieces to each other, which should achieve an effective corrosion protection in the contact zone. In particular, the sheet can be a hardenable sheet metal composed of steel with the general formula 22 MnB5, which is in particular coated with aluminum. It should be possible here to achieve strengths of 1300 to 1600 MPa.

DE 42 31 213 A1 has disclosed a process for producing a shaped component that is produced by means of pressing or deep-drawing; this shaped component, for example a structural member, is embodied with thicker walls in its middle section and with regions having reduced-thickness walls and in this case, is manufactured out of a one-piece sheet metal component that has a thickness corresponding to the greatest wall thickness of the shaped component to be produced and before the pressing or deep-drawing process, is reduced to the desired lesser thickness through rolling or a corresponding stretching process of another kind only in those regions in which the shaped component should have a lesser wall thickness.

DE 10 2004 054 795 A1 has disclosed a process for producing sheet metal components and vehicle body components in which at least one sheet based on a boron-alloyed case-hardened or quenched and tempered steel is joined to at least one sheet of approximately similar material quality or composed of another steel material and the material composite is subjected to at least one shaping process; at least the boron-alloyed sheet is hot formed and, with the shaping die halves closed, is subjected to an in-situ press hardening.

DE 10 2004 038 626 B3 has disclosed a process for producing hardened sheet steel components in which a component that has already been cut to its final contour and final hole pattern is heated and then inserted into a die in which only the outer edges are clamped while the entire component is cooled in the die and inside the die, comes to rest against the die as a result of the cooling. This hardens the component, which is composed of a hardenable steel.

The object of the invention is to create a process for producing sheet steel components with differing ductility and good corrosion protection.

SUMMARY OF THE INVENTION

According to the invention, a so-called patched sheet metal component is produced; this patched sheet metal component is produced either in that at least two sheets are placed onto each other and joined, formed into their final shape, and then subjected to an austenitizing step, which is followed by a quench hardening (indirect process) or in that the at least two sheets are joined to each other and then heated and shaped together.

According to the invention, the sheets are galvanized steel sheets, which, even after the heating and cooling, demonstrate a good cathodic corrosion protection that remains effective even without further after-treatment.

Naturally, other metallic coatings such as zinc alloys, aluminum, aluminum alloys, nickel, and nickel alloys are also suitable as coatings and are hereby disclosed as being equivalent.

According to the invention, it is thus possible to adjust the component properties in an extremely wide variety of ways.

1. Reducing austenitizing time enough to produce lower strengths and sufficient cathodic corrosion protection in the patched region, while a full austenitization occurs in the remaining regions.

2. Providing enough austenitizing time for complete through-hardening of a plate with a galvanized starting material of 22MnB5 connected to a plate component B composed of a respectively galvanized soft deep-drawing steel, a micro-alloyed steel, a carbon-manganese steel, or a dual-phase steel while producing a sufficient cathodic corrosion protection during the press hardening.

3. Separating the patches after the cold forming or press hardening.

4. Providing enough austenitizing time for complete through-hardening of plates A and B while producing a sufficient cathodic corrosion protection.

5. Shaping the patched plates in a hot forming die, with austenitization variants and material pairings as described in section 1, section 3, and section 4 while producing a sufficient cathodic corrosion protection and with the laser cutting or hard cutting with tools to be carried out after the hot forming.

The invention will be explained by way of example below in conjunction with the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the joined starting plates A and B before the cold forming and cold cutting.

FIG. 2 shows the component, composed of plates A and B, that has been cold formed in dies and cut to its final length.

FIG. 3 shows the component at a perforating station in which the welding spots are punched out, thus separating component A and component B from each other, either before or after the press hardening.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention offers several process options.

In process option 1, the cold-produced component (FIG. 2) composed of the joined individual plates A and B is heated in the furnace at approx. 900° C. until the larger component A has reached the austenitizing temperature in the regions in which it is not resting against the smaller component B.

In the regions in which components A and B are joined, the component has a higher mass to be heated.

This makes it possible, with the same furnace residence time, for region B of the component to not reach the austenitizing temperature while region A is heated to the austenitizing temperature due to its single sheet thickness and therefore lower mass.

The component is then cooled and press hardened in a press hardening die.

After the press hardening, the component has a more ductile material structure in region B because it did not reach the austenitizing temperature in the furnace.

Depending on the selected furnace residence time, it is possible to achieve a tensile strength of between 500 and 1600 MPa for the patched region and a tensile strength of between 1300 and 1600 MPa for the non-patched region.

The transitions from the non-austenitized regions to the austenitized regions are between 5 and 30 mm depending on the sheet thicknesses of the plates used, which avoids the occurrence of stress peaks in these regions. These soft transitions avoid undesirable stress peaks in the event of a vehicle crash.

Option 2:

In process option 2, the cold-produced component (FIG. 2) composed of the assembled individual plates A and B is heated in the furnace at approx. 900° C. until the component A has reached the austenitizing temperature. In this application, the base material of component B can be a soft deep-drawing steel, microalloyed steel, carbon-manganese steel, or a dual phase steel.

Then the component is cooled/press hardened in a press hardening die.

After the press hardening, the assembled component A has thus been converted into a martensitic structure in all regions and has a tensile strength of between 1300 and 1600 MPa in all regions.

Depending on the selection of the base material for component B, between 250 and 350 MPa is achieved in a soft deep-drawing steel, between 450 and 700 MPa is achieved in a micro-alloyed steel, between 500 and 750 MPa is achieved in a carbon-manganese steel, or between 700 and 1100 MPa is achieved in a dual-phase steel.

Due to the galvanized surfaces in component A and component B, the cathodic corrosion protection is achieved in all applications.

The material properties of component B also provide a wider process window with regard to furnace residence times.

Option 3:

In process option 3, the component shown in FIG. 2, which is composed of plates A and B and has been cold formed in forming dies and cut to its final size, is immediately separated again. See FIG. 3.

This can be carried out by punching out the welding spots in a perforation tool or by cutting out the welding spots using laser cutting or drilling.

Then, components A and B, which are now separate from each other, can each be heated separately to the respective austenitizing temperature in the furnace and subsequently cooled (press hardened) in a press hardening die.

However, it is also possible for the components A and B to be separated from each other only after the joint press hardening and for this separation to be carried out, for example, by lasers, hard perforation, or drilling and then for them to be supplied to further processing steps in the production of the vehicle body.

Option 4:

In process option 4, the cold-produced component (FIG. 2) composed of the assembled individual plates A and B is heated in the furnace at approx. 900° C. until the components A and B have reached the austenitizing temperature.

The component is then cooled/press hardened in a press hardening die.

After the press hardening, the assembled component composed of A and B has thus been converted into a martensitic structure in all regions and hardened; in all regions, it has a tensile strength of between 1300 and 1600 MPa and a good cathodic corrosion protection.

Option 5:

Shaping of a patched plate in a hot forming die, with austenitization variants and material pairings as described in section 1, section 2, and section 4.

After the hot forming, the component can be cut to achieve the desired final contour, either by laser or by hard cutting with tools.

A good cathodic corrosion protection is achieved in component A and component B, even in the patched region.

Advantages of the processes:

Option 1 and option 2:

-   -   For the production of 2 components, the cold forming and press         hardening each require only 1 tool set (cost savings)     -   Due to the zinc coating of the starting material, after the         heating and press hardening of the components, an outstanding         cathodic corrosion protection is produced even in the assembled         regions, as a result of which these regions do not require any         additional sealing steps     -   Weight savings since an increased sheet thickness of the         component is only required in the assembled regions     -   In the patched component region, it is possible to achieve more         ductile material properties because the austenitizing         temperature is not reached     -   No stress peaks in the transition region

One potential use for options 1 and 2 would be, for example, in an A pillar; in the region in which the door hinge is connected, it is advantageous to provide an increased sheet thickness, but also a more ductile material property.

Option 3

-   -   For the production of 2 components, the cold forming requires         only 1 tool set (cost savings)

Option 4:

-   -   For the production of 2 components, the cold forming and press         hardening each require only 1 tool set (cost savings)     -   Due to the zinc coating of the starting material, after the         heating and press hardening of the components, an outstanding         cathodic corrosion protection is produced even in the assembled         regions, as a result of which these regions do not require any         additional sealing steps     -   Increased sheet thickness only in the assembled regions and         consequently an increase in the load-bearing capacity in         selected areas with a minimal increase in weight     -   No stress peaks in the transition region

Option 5:

-   -   Due to the zinc coating of the starting material, after the         heating and hot forming of the components, an outstanding         cathodic corrosion protection is produced even in the assembled         regions, as a result of which these regions do not require any         additional sealing steps     -   The press hardening of the components requires only one press         hardening die     -   An increased sheet thickness only in the assembled regions and         consequently an increase in the load-bearing capacity in         selected areas with a minimal increase in weight     -   No stress peaks in the transition region     -   Achievable material properties as described in section 1,         section 2, and section 4 

1. A process for producing sheet steel components having regions of differing ductility, comprising: using a sheet metal plate comprising a hardenable steel alloy either to a) produce a component by of deep-drawing and then at least partially austenitizing the deep-drawn component using a heat treatment and subsequently quench hardening the deep-drawn component in a die; or b) at least partially austenitizing the plate using a heat treatment and shaping the plate in a hot state, and or subsequently quench hardening the plate; wherein the sheet metal plate has a zinc-based cathodic corrosion protection coating; and in regions of a desired higher ductility of the component, attaching at least one additional sheet to the plate, situated so that during the heat treatment, the plate is heated to a lesser degree there than in the remaining region.
 2. The process as recited in claim 1, comprising attaching the at least one additional sheet to the plate by welding or riveting.
 3. The process as recited in claim 1, characterized in that comprising removing the at least one additional sheet after the shaping and hardening.
 4. The process as recited in claim 1, wherein after the shaping and hardening, the at least one additional sheet remains on the plate as a material reinforcement.
 5. The process as recited in claim 1, wherein the at least one additional sheet is composed of the same steel material as the plate.
 6. The process as recited in claim 1, wherein the at least one additional sheet comprises at least one of the group consisting of a non-hardenable steel material, a soft deep-drawing steel, a micro-alloyed steel, a carbon-manganese steel, and a dual-phase steel. 